An insult to who? To you? I’ve been Catholic all my life. I don’t think rosaries should be worn around the neck. I don’t hang one from my mirror either, but I don’t see anything wrong with it. Why do Catholics fast, and a million other traditions? One reason is to keep us in the right frame of mind. If someone wants to hang a rosary from their mirror to feel better about themselves, I don’t see the harm.
I’m curious, do you feel the same way about people who put plastic Jesus or Madonna icons on their dash board? Again, it’s not something that I do, but I certainly would never feel offended by it.